Site selection criteria for the disposal of spent nuclear fuel in Romania

 Common opinion today is that the only practicable way for the disposal of hazardous waste is in deep-laying bedrock deposits. A set of general site selection criteria for spent nuclear fuel were developed, and are presented and discussed in this paper. Four widespread geological formations in Romania appropriate as geological barriers for underground disposal (salt, granite, volcanic tuff and green schists) were analyzed, and sites were proposed on the basis of the geological criteria defined for each of the four formations. Received: 18 July 1996 · Accepted: 27 September 1996     

Geochemical modelling of the weathering zone of the “Mina Fe” U deposit (Spain): A natural analogue for nuclear spent fuel alteration and stability processes in radwaste disposal

The “Mina Fe” U deposit (Salamanca, Spain) has been studied in the context of Enresa’s programme for U-mine sites restoration and also as a natural analogue for processes in high-level nuclear waste (HLNW) geological disposal. The investigations encompassed an array of geoscience disciplines, such as structural geology, mineralogy, hydrogeology and elemental and isotopic geochemistry and hydrogeochemistry of the site. Based on the obtained results, a conceptual mineralogical and geochemical model was performed integrating the main geochemical processes occurring at the site: the interaction between oxidised and slightly acidic water with pyrite, pitchblende, calcite and dolomite, as essential minerals of the U fracture-filling mineralisation, and hydroxyapatite from the host rock, as the main source of P. This conceptual model has been tested in a systematic numerical model, which includes the main kinetic (pyrite and pitchblende dissolution) and equilibrium processes (carbonate mineral dissolution, and goethite, schoepite and autunite secondary precipitation). The results obtained from the reactive-transport model satisfactorily agree with the conceptual model previously established. The assumption of the precipitation of coffinite as a secondary mineral in the system cannot be correctly evaluated due to the lack of hydrochemical data from the reducing zone of the site and valid thermodynamic and kinetic data for this hydrated U(IV)-silicate. This precipitation can also be hampered by the probable existence of dissolved U(IV)-organic matter and/or uranyl carbonate complexes, which are thermodynamically stable under the alkaline and reducing conditions that prevail in the reducing zone of the system. Finally, the intense downwards oxic and acidic alteration in the upper part of the system is of no relevance for the performance assessment of a HLNW disposal. However, the acidic and oxidised conditions are quickly buffered to neutral–alkaline and reducing at very shallow depths, of relevance for the performance assessment of a HLNW repository, even in a natural or artificially perturbed geological environment as “Mina Fe”.     

Evaluation of mineral precipitation potential in a spent nuclear fuel repository

In this study, the potential for mineral precipitation reactions to occur during the excavation, disposal, backfilling and closure of a deep geological repository for the final disposal of spent nuclear fuel was evaluated with the assistance of hydrogeochemical modelling. Four modelling exercises, corresponding to the main expectable geochemical scenarios in the Excavation Damaged Zone (EDZ) throughout repository evolution, were carried out and the times for sealing of the discontinuities owing to mineral precipitation in each of them were evaluated and discussed. The simulations show that mineral precipitation reactions are thermodynamically feasible in most of the studied cases. The main mineral phases potentially responsible for the hydraulic sealing of the EDZ are calcite and ferric oxyhydroxides, being the estimated volumes occupied by the precipitation of calcite between one and three orders of magnitude larger than the volume of precipitating ferric phases. The estimated times for complete sealing of the EDZ may vary between several hundreds to more than 1 million years. The shortest sealing times (less than 3,000 years) are obtained for the mixture of groundwaters from the repository depth with dilute infiltration waters.     

The uranium ore from Mina Fe (Salamanca, Spain) as a natural analogue of processes in a spent fuel repository

In the frame of the ENRESA natural analogue programme, the uranium ore from the “Mina Fe” (Salamanca, Spain) has been studied as a natural analogue of radioactive spent fuel behaviour. This uranium mine is hosted in highly fractured schistose rocks, a geological setting that has not been envisaged in the Spanish options for radioactive waste burial. However, some analogies with the processes that might be involved in the evolution of these geological repositories suggested this investigation.

The pitchblende–pyrite–carbonate paragenesis has been studied “in situ” as natural analogue of the nuclear spent fuel behaviour under extremely oxidative dissolution conditions. Similarly, secondary Fe oxyhydroxides and clay minerals have also been considered as relevant analogue materials for the retention of uranium and other analogous trace metals. A multidisciplinary characterisation of the site has been performed in order to study these processes.

Though the intense mining activities in the site hindered precise determination of the original hydrogeological and hydrochemical features of the investigated zone (Boa fault zone), the mineralogy and geochemistry of fracture fillings, mineralisation and associated clayey materials have allowed the geochemical evolution of the system to be established. Three geochemical zones have been clearly differentiated: (i) the oxidised zone, from the surface to approximately 20 m depth, (ii) the redox transition zone, from 20 to 50 m depth, and (iii) the reduced zone, located below the transition zone.

The oxidised zone is characterised by the presence of the typical mineral association resulting from the strong acid conditions caused by the total oxidation of pyrite and other sulphides. The total oxidation, dissolution and leaching of U(IV), as uranyl–sulphate aqueous complexes, prevailed in this oxidised zone. The redox transition zone is characterised by the coexistence of the primary uranium paragenesis, oxidised minerals, as well as numerous secondary solid phases as a result of the physico-chemical changes in the environment. The optimal physico-chemical conditions for the coffinitisation of pitchblende and the co-precipitation of Fe(III)–U(VI) took place in this zone. In the reduced zone, where the primary uranium paragenesis is present, we currently find the necessary physico-chemical conditions to stabilise pitchblende, pyrite and carbonates.

The physico-chemical conditions of the oxidised zone are not relevant to disposal conditions. In the transition zone, two main geochemical processes take place: (i) the coffinitisation of pitchblende, which may be an important process for the stability of spent fuel in reducing conditions, and (ii) the co-precipitation of the Fe(III) and U(VI) as oxyhydroxides, another relevant mechanism for the retention of uranium. The physico-chemical conditions that prevail below 50 m depth should be sufficient to stabilise a spent nuclear fuel repository, in the same way as they have been able to preserve the 34-Ma-old uranium deposit of the Mina Fe.     

A natural analogue of nuclear waste glass in compacted bentonite

A marine based argillaceous rock containing volcanic glass shards has been investigated to infer the long-term durability of vitrified nuclear waste in compacted bentonite, which is a candidate for buffer material constituting the engineered barrier system for nuclear waste disposal. Fission track ages indicate that the volcanic glass shards, andesitic scoria, have been buried in the argillaceous rock for about 1 Ma. Neither glass matrix dissolution nor precipitation on the surface was seen under an optical microscope. Little leaching of any element has been recognized by analyses using an electron microprobe analyzer. Secondary ion mass spectrometry analysis, however, indicates significant hydration which may dominantly be a permeation of molecular water.As an indicator of durability of glass against groundwater a normalized mass loss of Si (NL_Si) has been evaluated for the volcanic glass based on free energy for hydration. The difference between estimated NL_Si of the volcanic glass and that of a simulated waste glass is within one order so that the volcanic glass may be analogous to a waste glass with respect to durability to water. The argillaceous rock is analogous to the compacted bentonite with respect to physical properties such as dry-density, unconfined compression strength, porosity, and hydraulic conductivity. The ambient physical and chemical conditions surrounding the volcanic glass have been also investigated: temperature was in the range from 4 to 30°C due to the burial history of the volcanic glass. Over most of the past 1 Ma the volcanic glass has been in contact with groundwater originating from seawater. Thermodynamic calculations indicate (1) pH (=7.74–7.94) of the groundwater has mainly been controlled by dissolution of carbonate minerals, (2) the redox potential (Eh=−34–−73 mV) of the groundwater has dominantly been controlled by decomposition of organic materials to produce CH₄(g), and (3) activity of aqueous silica of the groundwater was in equilibrium with SiO₂ amorphous. Because of the equilibrium between aqueous silica and SiO₂ amorphous, the volcanic glass did not dissolve during the burial.Vitrified nuclear waste sealed in compacted bentonite, therefore, will not dissolve significantly if buried in an environment as mentioned above.     

Implementation of microbial processes in the performance assessment of spent nuclear fuel repositories

Present strategies for the long-term disposal of high-level nuclear wastes are based on the construction of repositories hundreds of meters below the earth surface. Although the surrounding host-rocks are relatively isolated from the light at the earth surface they are by no means lifeless. Microorganisms rule the deep part of the biosphere and it is well established that their activity can alter chemical and physical properties of these environments. Microbial processes can directly and indirectly affect radionuclide migration in multiple ways. Within 6th FP IP FUNMIG the interplay between microbial biofilms and radionuclides and the effect of microbially induced redox transformations of Fe on radionuclide mobility have been investigated. For the first time, formation of U(V) as a consequence of microbial U(VI) reduction in a multi-species biofilm was detected in vivo by combining laser fluorescence spectroscopy and confocal laser scanning microscopy. Furthermore, it was demonstrated that addition of U(VI) can lead to increased respiratory activity in a biofilm. Increased respiration in a biofilm can create microenvironments with lower redox potential, and hence induce reduction of radionuclides. Transient mobilization of U was observed in experiments with Fe oxides containing adsorbed U(VI) in which the activity of SO₄-reducing organisms was mimicked by sulfide addition. Faster reaction of sulfide with Fe oxides compared to U(VI) reduction, and decreasing U(VI) adsorption due to the transformation of Fe oxides into FeS can explain the observed intermittent U mobilization. The presented research on microbe-radionuclide interactions performed within FUNMIG addresses only a few aspects of the potential role of microorganisms in the performance assessment of nuclear waste repositories. For this reason, additionally, this article provides a cursory overview of microbial processes which were not studied within the FUNMIG project but are relevant in the context of performance assessment. The following aspects are presented: (a) the occurrence and metabolic activity of microorganisms of several proposed types of host-rocks, (b) the potential importance of microorganisms in the near-field of nuclear waste repositories, (c) indirect effects of microbial processes on radionuclide mobility in the repository far-field, (d) binding of radionuclides to microbial biomass, (e) microbial redox transformations of radionuclides, and (f) the implementation of microbial processes in reactive transport models for radionuclide migration.     

A case study on the influence of THM coupling on the near field safety of a spent fuel repository in sparsely fractured granite

In order to demonstrate the feasibility of geological disposal of spent CANDU fuel in Canada, a safety assessment was performed for a hypothetical repository in the Canadian Shield. The assessment shows that the maximum long term radionuclide release from such repository would meet international criteria for dose rate; however, uncertainties in the assumed evolution of the repository were identified. Such uncertainties could be resolved by the consideration of coupled Thermal-Hydro-Mechanical-Chemical (THMC) processes. In Task A of the DECOVALEX-THMC project, THM models were developed within the framework of the theory of poroelasticity. Such model development was performed in an iterative manner, using experimental data from laboratory and field tests. The models were used to perform near-field simulations of the evolution of the repository in order to address the above-mentioned uncertainties. This paper presents the definition and rationale of task A and the results of the simulations. From a repository safety point of view, the simulations predict that the maximum temperature would be well below the design target of 100°C; however, the stress on the container can marginally exceed the design value of 15 MPa. However, the most important finding from the simulations is that a rock damage zone could form around the emplacement borehole. Such damage zone can extend a few metres from the walls of the emplacement holes, with permeability values that are orders of magnitude higher than the initial values. The damage zone has the potential to increase the radionuclide transport flux from the geosphere; the effect of such an increase should be taken into account in the safety assessment and mitigated if necessary by the provision of sealing systems. Prepared for publication in Environmental Geology. DECOVALEX-THMC Special Issue.     

Thermochemically induced transformations in Al-smectites: A Spanish natural analogue of the bentonite barrier behaviour in a radwaste disposal

The thermal effect induced by the Morrón de Mateo volcanic dome (Cabo de Gata volcanic region, Spain) on the adjacent bentonitised tuffaceous beds has been studied as a natural analogue of the thermal behaviour of the bentonite-engineered barrier of a geological radwaste repository. These bentonites consist mainly of Fe-rich smectites and were formed in equilibrium with seawater at temperatures between 75 and 95 °C, according to the δ¹⁸O and δD values. In contrast, bentonites from other localities in the region consist mainly of Al-smectites, formed in equilibrium with meteoric water below 25 °C.This investigation is focussed on the detection of the chemical differences between smectites from proximal and distal zones to the dome, as well as to test whether the temperatures calculated based on the O and H isotopic values correspond to their formation or transformation. The initial hypothesis was that the chosen smectites could be formed under marine conditions, being later transformed and isotopically re-equilibrated as a result of the intrusion. To check this hypothesis, a detailed mineralogical, chemical, geochemical and isotopic study has been performed on the smectitised tuffaceous materials and the overlaying biocalcarenites outcropping near and far from the dome.The results show that distal smectites are dioctahedral Al-smectites, similar to those from other deposits in the region, while proximal smectites are Fe- and Mg-rich smectites, showing two evolutionary trends on a Fe–Mg–Al ternary diagram. Similar features are observed when their structural formulae are plotted on the muscovite–celadonite–pyrophylite diagram. Thus, they plot in the smectite domain with interlayer charge less than 1, which is mainly due to octahedral substitution for distal smectites, while for proximal ones it is caused by both octahedral and tetrahedral substitutions. In this ternary diagram, the domains of both proximal and distal smectites are partially overlapped. The coexistence of di- and trioctahedral smectites was only detected in one proximal sample. Further, proximal biocalcarenites are enriched in Fe-rich dolomite in relation to the distal ones.The ⁸⁷Sr/⁸⁶Sr and δ¹³C values in carbonates and δD in smectites indicate equilibrium with seawater. In contrast, δ¹⁸O values of carbonates and smectites indicate that they were transformed and re-equilibrated between 40 and 90 °C, and between 55 and 66 °C, respectively, independently of their location with respect to the dome.These data suggest that the transformation of calcite into Mg–Fe-carbonates and the occurrence of Fe- and Mg-rich smectites near to the dome resulted from a chemically induced process at similar temperatures. The compositional differences among samples suggests that Fe, Mg and minor Mn were supplied by a contaminant plume originated from the dome, migrating through the sediments and becoming more diluted away from the source. The absence of a well-defined thermal gradient in the system could be due to the small size, semi-closed and shallow character of the basin, as well as to its high underlying volcanic activity.Finally, the results are discussed in terms of analogue processes that can be expected in the bentonite barrier of a radwaste geological repository.     

Vertical distribution, stability and evolution of groundwater composition in granite suitable for storage of spent nuclear fuel

Granite with extremely low permeability has been selected to host a repository of spent nuclear fuel in the Czech Republic. Three boreholes were drilled in a test site of the Podlesi granite stock in the Krusne hory Mts. The holes were located 10 m apart. After hydraulic tests, four sections in one of the boreholes were separated by packers at depth of 69-111, 111-161, 161-220 and 220-300 m. Samples of groundwater from each section were periodically collected for chemical and isotopic analysis. Groundwater from fractures in the granite stock does not belong to a single and uniform groundwater body in spite of that the granite is chemically and mineralogically homogeneous. There are three water bodies, which are only partly hydraulically connected. They are： （1） The groundwater in the oxidation zone to a depth of about 111 m. （2） The groundwater of the zone of hydrolysis of alumosilicates from 111 to 220 m. （3） The groundwater of the zone of hydrolysis of alumosilicates below the depth of 220 m from a different fracture system than the water from the above sections. The total dissolved solids of water increase with depth. The Ca-SO4 component predominates in the near surface water body while Na-HCO3 component predominates in the two deeper water bodies. Water from the oxidation zone contains higher concentrations of iron and other trace metals. The chemical composition of water in the three water bodies changed during the 14 months of sampling. No steady state was reached during this time. The changes displayed systematic trends. The ratio Ca/Na increased and the ratio HCO3/SO4 decreased with time in the shallow water body. In contrast, the ratio CafNa decreased and HCO3/SO4 fluctuated without an obvious trend in the deeper water bodies. An unusually high concentration of dissolved organic carbon （DOC） was found in the lowest section of the test borehole. The concentration of DOC was 150 mg/L at the beginning of sampling. The isotopic composition was δ^13C=-27.6 ‰. The concentration of DOC dropped both towards the surface and with time.     

A study of the compaction properties of potential clay—sand buffer mixtures for use in nuclear fuel waste disposal

In-situ emplacement of clay-based buffers in a nuclear fuel waste disposal vault limits the maximum attainable buffer density. This will vary with the composition of the buffer. A study of the maximum attainable densities of candidate Na bentonite/sand and illite/sand buffers is described. The addition of sand significantly increases the achievable compacted density. This increase may be obtained without any decrease in the swelling pressures developed by Na bentonite buffers. Sand decreases the shrinkage potential of the buffer and may also decrease the mass diffusion coefficient. A mixture of 50% sand and 50% clay by mass appears to optimise the physical properties of the buffer.     

Characterization of natural organic matter in bentonite clays for potential use in deep geological repositories for used nuclear fuel

The Nuclear Waste Management Organization (NWMO) is developing a Deep Geological Repository (DGR) to contain and isolate used nuclear fuel in a suitable rock formation at a depth of approximately 500 m. The design concept employs a multibarrier system, including the use of copper-coated used fuel containers, surrounded by a low-permeability, swelling clay buffer material within a low permeability, stable host rock environment. The natural organic matter (NOM) composition of the bentonite clays being considered for the buffer material is largely uncharacterized at the molecular-level. To gain a better understanding of the NOM in target clays from Wyoming and Saskatchewan, molecular-level methods (biomarker analysis, solid-state ¹³C NMR and solution-state ¹H nuclear magnetic resonance (NMR)) were used to elucidate the structure and sources of NOM. Organic carbon content in three commercially available bentonites analyzed was low (0.11–0.41%). The aliphatic lipid distribution of the clay samples analyzed showed a predominance of higher concentration of lipids from vascular plants and low concentrations of lipids consistent with microbial origin. The lignin phenol vanillyl acid to aldehyde ratio (Ad/Al) for the National sample indicated an advanced state of lignin oxidation and NOM diagenesis. The ¹³C NMR spectra were dominated by signals in the aromatic and aliphatic regions. The ratio of alkyl/O-alkyl carbon ranged from 7.6 to 9.7, indicating that the NOM has undergone advanced diagenetic alteration. The absence lignin-derived phenols commonly observed in CuO oxidation extracts from contemporary soils and sediments as well as the lack of amino acids suggests that the material corresponding to the aromatic signal is not composed of lignin or proteins but may be derived from another source such as black carbon or some other non-extractable aromatic-rich NOM. The aliphatic signal appears to correspond to long-chain compounds with little side branching based on the results of the one-dimensional (1D) and two-dimensional (2D) solution-state ¹H NMR analyses. Overall, the organic geochemical analyses suggest that the NOM is composed mainly of plant-derived waxes and highly aromatic carbon with low contributions from small molecules. The compounds identified by the molecular-level analysis of NOM in the clay samples are hypothesized to be recalcitrant but future studies should examine if these compounds may serve as a microbial substrate to further test the observations of this study. Furthermore, our study suggests that the NOM has undergone diagenesis and that marine NOM signatures are no longer recognizable or detectable. As such, future work may also examine the diagenesis of these deposits to further understand the NOM geochemistry and paleoenvironmental conditions in bentonite deposits.     

Problems of site selection for nuclear waste disposal in a loess-covered hill environment in Hungary

Problems of the long-term storage of nuclear waste produced by the Paks power plant have recently come to the fore in Hungary. After an extensive debate between investors and the local population the decision makers took the side of those having opposed the establishment of the waste disposal site in the initially proposed environment. Several studies have been conducted to support both pros and cons. Although the idea was rejected finally, this debate has proven that comprehensive research prior to decision making is indispensable in similar cases.Regretfully, the academic staff of the GRIHAS was invited to participate in the expertise too late, in 1988, to investigate the geomorphological and social-economic environmental conditions of the proposed site. The latter cannot be neglected since the settlement pattern, demographic structure, character of economic activities, the level and state of the infrastructure — though indirectly — exert a long-term impact on the circumstances of the operation of the site, the living conditions of the people working there and on the use of the environment. Any harmful effect from the secondary wastes to be deposited could endanger the security of the local population, that is why a thorough geographical analysis based on field research in the immediate and wider surroundings of the site is necessary already in the phase of decision preparation.     

Chemical and isotopic characterization of water–rock interactions in shales induced by the intrusion of a basaltic dike: A natural analogue for radioactive waste disposal

Disposal of nuclear waste in deep geological formations is expected to induce thermal fluxes for hundreds of years with maximum temperature reaching about 100–150 °C in the nearfield argillaceous environment. The long-term behavior of clays subjected to such thermal gradients needs to be perfectly understood in safety assessment considerations. In this respect, a Toarcian argillaceous unit thermally disturbed by the intrusion of a 1.1-m wide basaltic dike at the Perthus pass (Herault, France), was studied in detail as a natural analogue. The thermal imprint induced by the dike was evaluated by a mineralogical, chemical and K–Ar study of the <2 μm clay fraction of shale samples collected at increasing distance from the basalt. The data suggest that the mineral composition of the shales was not significantly disturbed when the temperature was below 100–150 °C. Closer to the dike at 150–300 °C, changes such as progressive dissolution of chlorite and kaolinite, increased content of the mixed layers illite–smectite with more illite layers, complete decalcification and subsequent increased content of quartz, were found.At the eastern contact with the dike, the mineral and chemical compositions of both the shales and the basalt suggest water–rock interactions subsequent to the intrusion with precipitation of palagonite and renewed but discrete deposition of carbonate. A pencil cleavage developed in the shales during the dike emplacement probably favored water circulation along the contact. Strontium isotopic data suggest that the fluids of probable meteoric origin, reacted with Bathonian and Bajocian limestones before entering the underlying Toarcian shales.By analogy with deep geological radioactive waste repositories, the results report discrete mineralogical variations of the clays when subjected to temperatures of 100–150 °C that are expected in deep storage conditions. Beyond 150 °C, significant mineralogical changes may alter the physical and chemical properties of the shales, especially of the clay fraction. Also, the development of structural discontinuities in the so-called thermally disturbed zone might be of importance as these discontinuities might become zones for preferential fluid circulation. Finally, the study emphasizes the use of Rb–Sr and K–Ar isotopic systems as tracers of local circulating fluids related to low-grade thermal imprints.     

A natural cement analogue study to understand the long-term behaviour of cements in nuclear waste repositories: Maqarin (Jordan)

The geological storage of nuclear waste includes multibarrier engineered systems where a large amount of cement-based material is used. Predicting the long term behaviour of cement is approached by reactive transport modelling, where some of the boundary conditions can be defined through studying natural cement analogues (e.g. at the Maqarin natural analogue site). At Maqarin, pyrometamorphism of clay biomicrites and siliceous chalks, caused by the in-situ combustion of organic matter, produced various clinker minerals. The interaction of infiltrating groundwater with these clinker phases resulted in a portlandite-buffered hyperalkaline leachate plume, which migrated into the adjacent biomicrite host rock, resulting in the precipitation of hydrated cement minerals.In this study, rock samples with different degrees of interaction with the hyperalkaline plume were investigated by various methods (mostly SEM-EDS). The observations have identified a paragenetic sequence of hydrous cement minerals, and reveal how the fractures and porosity in the biomicrite have become sequentially filled. In the alkaline disturbed zone, C-A-S-H (an unstoichiometric gel of Ca, Al, Si and OH) is observed to fill the pores of the biomicrite wallrock, as a consequence of reaction with a high pH Ca-rich fluid circulating in fractures. Porosity profiles indicate that in some cases the pores of the rock adjacent to the fractures became tightly sealed, whereas in the veins some porosity is preserved. Later pulses of sulphate-rich groundwater precipitated ettringite and occasionally thaumasite in the veins, whereas downstream in the lower pH distal regions of the hyperalkaline plume, zeolite was precipitated.Comparing our observations with the reactive transport modelling results reveals two major discrepancies: firstly, the models predict that ettringite is precipitated before C-A-S-H, whereas the C-A-S-H is observed as the earlier phase in Maqarin; and, secondly, the models predict that ettringite acts as the principal pore-filling phase in contrast to the C-A-S-H observed in the natural system. These discrepancies are related to the fact that our data were not available at the time the modelling studies were performed. However, all models succeeded in reproducing the porosity reduction observed at the fracture–rock interface in the natural analogue system.     

Principles of geomechanical safety assessment for radioactive waste disposal in salt structures

Today, a large amount of knowledge is available concerning various sites of potential high active waste (HAW) repositories in salt media. Domal Zechstein salt formations have been examined at several sites in Germany. Extensive R&D work was initiated in the former Asse Salt Mine in order to explore the suitability of salt for waste isolation by laboratory tests, theoretical studies and in-situ tests with test results forming a technological base for future repository development.

Resulting from the inhomogeneity of salt structures the demanded safety of a permanent repository for radioactive wastes can be demonstrated only by a specific site analysis in which the entire system, “the geological situation, the repository, and the form and amount of the wastes” and their interrelationships are taken into consideration.

The site analysis has three essential tasks: (1) Assessment of the thermomechanical load capacity of the host rock, so that deposition strategies can be determined for the site; (2) Determination of the safe dimensions of the mine (e.g. stability of the caverns and safety of the operations); and (3) Evaluation of the barriers and the long-term safety analysis for the authorization procedure.

The geotechnical stability analysis is a critical part of the safety assessment. Engineering–geological study of the site, laboratory and in situ-experiments, geomechanical modelling, and numerical static calculations comprise such an analysis.

Within a scenario analysis — according to the multi-barrier principle, the geological setting is checked to be able to contribute significantly to the waste isolation over long periods. The assessment of the integrity of the geological barrier can only be performed by making calculations with geomechanical and hydrogeological models. The proper idealization of the host rock in a computational model is the basis of a realistic calculation of stress distribution and excavation damage effects. The determination of water permeability along discontinuities is necessary in order to evaluate the barrier efficiency of each host rock.

In this paper some important processes for the performance assessment are described, namely creep and fracturing, permeability and infiltration, and halokinesis and subrosion.

For the future, the role and contributions of geoscientific and rock mechanics work within the safety assessment issues (e.g. geomechanical safety indicators) must be identified in greater detail, e.g. considerations of geomechanical natural analogy for calibration of constitutive laws.     

Inferences from a corrosion study of a bronze cannon,applied to high level nuclear waste disposal

A bronze cannon was used as an analogue of corrosion processes for Cu. The cannon has been embedded in clay sediments in the Baltic Sea since 1676. Corrosion products are cuprite and malachite, mainly derived from transformation of tenorite inclusions of the bronze alloy. The bronze matrix exhibits little corrosion and a conservative estimate for the maximum corrosion of Cu is <10 mm in 100,000 a. Mechanisms for the chemical corrosion processes are suggested.